1. Technical Field
The present invention is directed to a method for increasing the maximum operating speed of an internal combustion engine operated in a low temperature combustion mode such as a homogeneous charge compression ignition mode.
2. Description of the Related Art
Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. Internal combustion engines, especially automotive internal combustion engines, generally fall into one of two categories, spark ignition engines and compression ignition engines. Traditional spark ignition engines, such as gasoline engines, typically function by introducing a fuel/air mixture into the combustion cylinders, which is then compressed in the compression stroke and ignited by a spark plug. Traditional compression ignition engines, such as diesel engines, typically function by introducing or injecting pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke. Traditional gasoline engine combustion results in a premixed turbulent flame, while traditional diesel engine combustion results in a mixing controlled diffusion flame. Both processes are controlled by fluid mechanics, as well as heat and mass transfer. Each type of engine has advantages and disadvantages. In general, gasoline engines coupled with 3-way emissions catalysts produce fewer emissions but are less efficient, while, in general, diesel engines are more efficient but produce more emissions.
These air pollutants are composed of particulates and gaseous compounds including, among other things, oxides of nitrogen (NOx). Continued global emphasis and government legislation on reducing emissions and improving fuel economy of internal combustion engines has led to the need to develop advanced high efficiency, clean combustion engines. Exhaust after-treatment systems (such as Selective Catalyst Reduction (SCR), lean NOx traps, and diesel particulate filters) have been designed and commercialized to lower exhaust emissions of diesel engines to meet emission targets and regulations. However, these systems are costly, add to the weight of the vehicle, and minimize fuel economy due to the added weight, increase in exhaust back pressure, and the need to use fuel to regenerate the systems. Reducing engine-out emissions would decrease the size and/or eliminate the need for these systems.
One approach explored in the industry to simultaneously reduce emissions (compared to a traditional diesel engine) and improve efficiency (compared to a traditional gasoline engine) is to operate the engine at a lower combustion temperature (typically called “low temperature combustion” (LTC)). This can be achieved by premixing some or all of the fuel with the air (and recycled exhaust gas) prior to entering the cylinder, or alternatively, in the cylinder, well before combustion occurs. This, in turn, greatly reduces (or eliminates) the fraction of the fuel that is burned with a mixing controlled diffusion flame (diffusion flames lead to high combustion temperatures). Also, the ratio of the fuel to the total mass in the cylinder is kept low to ensure low temperature combustion. This does greatly reduces the effectiveness of a spark plug. As a result, ignition is normally initiated via compression; however, a spark plug can be used to assist. The low temperature after combustion significantly reduces NOx formation, due to the fundamental chemistry of the reaction pathway. The use of fuels with gasoline-type volatilities (vs. heavier fuels such as diesel), combined with premixing the fuel and air, limit soot production. Operating with LTC also improves engine efficiency by reducing heat losses, and by allowing the optimization of various engine parameters. There are several types of LTC each of which has a distinct acronym for example: HCCI, PCCI, RCCI, CAI, PPC, MK, UNIBUS, OKP, and the like. One drawback to these LTC-type technologies is that the speed-load (power output) operating range is very limited, and significantly smaller than required and provided for by current gasoline spark-ignited and diesel compression ignition internal combustion engines.
One reason the speed-load range is limited with LTCengines is that it is difficult to control the ignition timing. For example, with respect to the speed range, as the engine speed increases, the fuel has less time to ignite. This is a larger concern for advanced combustion engines since ignition is initiated via compression, and is not initiated solely with a spark plug. Accordingly, as the speed increases in advanced combustion engines, the possibility that the fuel will not ignite (misfire) increases, and the engine becomes unstable (a higher combustion variance). Consequently, the combustion variance dictates the maximum allowable speed for advanced combustion engines.
Ongoing R&D efforts have shown that fuel compositions and their properties can have an impact on the speed-load range that can be obtained. For example, U.S. Patent Application Publication Number 2011/0271925 (“the '925 application”) discloses fuel compositions that yield very low soot and low NO emissions while having high efficiencies and acceptable maximum in-cylinder pressure rise rates over a wide load range when used in an advanced combustion engine environment, especially one operating in partially-premixed combustion (PPC) mode. The fuel compositions disclosed in the '925 application have a boiling range of between 95 to 440 degrees Fahrenheit, and (a) a total sum of n-paraffins and naphthenes content of at least 7 volume percent and (b) a preferred RON of about 80 or less.
Another example is U.S. Patent Application Publication Number 20120012087 (“the '087 application”) which discloses fuel compositions that provide: (a) a significant reduction in NOx, (b) a reduction in soot emissions, and (c) high efficiencies, especially when compared to conventional diesel fuel compositions, when the fuels of that invention are employed in a partially premixed combustion mode in an advanced combustion engine. The fuel compositions disclosed in the '087 application have a boiling range of between 95 to 440 degrees Fahrenheit, and (a) a total sum of n-paraffins and naphthenes content of at least 22 volume percent and (b) a RON of about 90 or less. The best performing fuels had a RON of 80 or less.
A significant drawback of the use of fuels such as naphthas having lower octane than pump gasoline is that they are present in refineries in much smaller quantities than gasoline and availability for sale at fuel stations would require additional fuel storage tanks which most fuel stations do not have space for. It would therefore be more advantageous, and cost effective, if the conventional pump gasoline could also be formulated to work in these engines. However, due to the higher RON of conventional pump gasoline, the speed range may be affected. It generally takes a longer time to compression ignite higher RON fuels, which can become an issue at higher speeds.
One approach is to use additives to change the reactivity of pump gasoline. Cetane improvers such as 2-ethylhexyl nitrate (EHN) and di-tert butyl peroxide (DTBP) have typically been added to diesel fuels to increase their cetane number. However, the use of cetane improvers in conventional pump gasolines is limited, particularly in LTC processes. For example, SAE Paper 2003-01-3170 by Eng et. al. discloses the use of DTBP to lower the low load stability limit in an HCCI single cylinder engine operated with PRF85 (a mixture of 85% iso-octane and 15% n-heptane, which by definition has a RON=MON=(RON+MON)/2=85). These types of PRF's are frequently used in research to represent gasoline. However, gasoline is known to be a more complex mixture and does not always perform the same as PRF. Further, they state that “adding an ignition promoter to extend the lower fueling rate limit” (i.e. the low load limit) “will result in a corresponding decrease in the maximum fueling level” (i.e. the high load limit).
Another example is SAE paper 2011-01-0361 by Hanson et. al which discloses the addition of EHN to gasoline to lower the low load limit in Reactivity-Controlled Compression Ignition (RCCI). RCCI utilizes two fuels with different reactivities and multiple fuel injections (one port and the other direct injection to create some stratification) to control air-fuel mixture reactivity in engine cylinders.
Combustion and Flame publication (132, (2003), 291-239) by Tanaka et. al. added 0.5 to 2% DTBP and EHN to PRF90 (90% iso-octane+10% n-heptane) and tested fundamental combustion behavior in a rapid compression machine (not an engine). Tanaka et. al. found that the cetane improvers shortened the ignition delay time (i.e., speed up the start of combustion). In addition, Tanaka et. al, reported that DTBP is more effective than EHN.
Heretofore, there has been no appreciation or recognition that the addition of one or more cetane improvers to conventional pump gasoline can increase or expand the engines range of operating speeds and thus improve its performance and feasibility for use in advanced combustion engines.